# std::unordered_set<Key,Hash,KeyEqual,Allocator>::emplace_hint

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< cpp‎ | container‎ | unordered set

template< class... Args > iterator emplace_hint( const_iterator hint, Args&&... args ); |  | (since C++11)

Inserts a new element into the container, using hint as a suggestion where the element should go.

The constructors of the key and mapped value are called with exactly the same arguments as supplied to the function, forwarded with std::forward<Args>(args)....

If after the operation the new number of elements is greater than old `max_load_factor()``*``` `bucket_count()` a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.

### Parameters

hint | - | iterator, used as a suggestion as to where to insert the new element
args | - | arguments to forward to the constructor of the element

### Return value

An iterator to the inserted element, or to the element that prevented the insertion.

### Exceptions

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee).

### Complexity

Amortized constant on average, worst case linear in the size of the container.

### Example

Run this code

    #include <chrono>
    #include <cstddef>
    #include <functional>
    #include <iomanip>
    #include <iostream>
    #include <unordered_set>

    const int n_operations = 100'500'0;

    std::size_t set_emplace()
    {
        std::unordered_set<int> set;
        for (int i = 0; i < n_operations; ++i)
            set.emplace(i);
        return set.size();
    }

    std::size_t set_emplace_hint()
    {
        std::unordered_set<int> set;
        auto it = set.begin();
        for (int i = 0; i < n_operations; ++i)
        {
            set.emplace_hint(it, i);
            it = set.end();
        }
        return set.size();
    }

    std::size_t set_emplace_hint_wrong()
    {
        std::unordered_set<int> set;
        auto it = set.begin();
        for (int i = n_operations; i > 0; --i)
        {
            set.emplace_hint(it, i);
            it = set.end();
        }
        return set.size();
    }

    std::size_t set_emplace_hint_corrected()
    {
        std::unordered_set<int> set;
        auto it = set.begin();
        for (int i = n_operations; i > 0; --i)
        {
            set.emplace_hint(it, i);
            it = set.begin();
        }
        return set.size();
    }

    std::size_t set_emplace_hint_closest()
    {
        std::unordered_set<int> set;
        auto it = set.begin();
        for (int i = 0; i < n_operations; ++i)
            it = set.emplace_hint(it, i);
        return set.size();
    }

    double time_it(std::function<std::size_t()> set_test,
                   const char* what = nullptr,
                   double ratio = 0.0)
    {
        const auto start = std::chrono::system_clock::now();
        const std::size_t setsize = set_test();
        const auto stop = std::chrono::system_clock::now();
        const std::chrono::duration<double, std::milli> time = stop - start;
        if (what != nullptr && setsize > 0)
            std::cout << std::setw(8) << time << " for " << what << " (ratio: "
                      << (ratio == 0.0 ? 1.0 : ratio / time.count()) << ")\n";
        return time.count();
    }

    int main()
    {
        std::cout << std::fixed << std::setprecision(2);
        time_it(set_emplace); // cache warmup
        const auto x = time_it(set_emplace, "plain emplace");
        time_it(set_emplace_hint, "emplace with correct hint", x);
        time_it(set_emplace_hint_wrong, "emplace with wrong hint", x);
        time_it(set_emplace_hint_corrected, "corrected emplace", x);
        time_it(set_emplace_hint_closest, "emplace using returned iterator", x);
    }

Possible output:

    146.88ms for plain emplace (ratio: 1.00)
    168.20ms for emplace with correct hint (ratio: 0.87)
    168.78ms for emplace with wrong hint (ratio: 0.87)
    166.58ms for corrected emplace (ratio: 0.88)
    168.27ms for emplace using returned iterator (ratio: 0.87)

### See also

emplace | constructs element in-place (public member function)
insert | inserts elements or nodes(since C++17) (public member function)

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